1. Field of the invention
This invention relates generally to adjusting the volume of ink droplets ejected from a printhead, and more particularly to precisely measuring the temperature of heater elements of an ink-jet printhead.
2. Description of prior art
Ink-jet printers of the type usually referred to as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, have at least one printhead from which droplets of ink are directed towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. For a drop-on-demand printhead, power pulses cause the droplets of ink to be expelled as required from nozzles or orifices at the end of the channels. In a thermal printer, the power pulses are usually produced by formation and growth of vapor bubbles on heating elements or resistors, each located in a respective channel, which are individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially expels the ink therein from the channel orifice, thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium where, upon hitting the recording medium, a dot or spot of ink is deposited. Following collapse of the vapor bubble the channel is refilled by ink from a supply container of liquid ink.
The uniformity of the ink jet significantly affects the printing quality, especially for a high-resolution printhead. The volume of the ink droplet depends on the applied voltage, as well as the initial temperature of heating elements. The circumference of a heating element and the duration of its use influence its temperature, which will rise while continuously ejecting the ink droplets. Therefore, adjusting the temperature of the heating elements can change the driving manner of an ink-jet printhead.
Refer to FIG. 1. In the prior-art, the average temperature of a thermal ink-jet chip 10 is measured. In the drawing, reference number 102 indicates an electrode area, reference number 103 indicates a temperature-sensing resistor, reference number 104 indicates an ink-jet circuit area, reference number 105 indicates a driving device area, reference number 106 indicates a heating elements area, and reference number 107 indicates an opening of supply container of liquid ink.
Regarding the sensing and control of temperature, the prior art, e.g. U.S. Pat. No. 4,791,435, U.S. Pat. No. 4,910,528, U.S. Pat. No. 5,107,276, U.S. Pat. No. 5,168,284, U.S. Pat. No. 5,175,565, U.S. Pat. No. 5,475,405, U.S. Pat. No. 5,736,995 and soon, only consider the average temperature. In all of the prior arts, heating elements are pre-heated to keep above a certain temperature in accordance with the feedback of the average temperature of the ink-jet chip when the heating elements are used. Furthermore, the ink-jet printer is paused to cool the printhead down if the average temperature gets too high as described in U.S. Pat. No. 4,910,528. However, only considering the average temperature is not enough for the printhead since each of the individual heating elements might be used for different durations.
In a high-resolution printhead, there are many heating elements. Driving circuits such as switching circuits of MOS transistors are usually formed on the ink-jet chip as described in U.S. Pat. No. 5,045,870 and U.S. Pat. No. 5,211,812. Refer to FIG. 2, which is a cross-sectional diagram of a MOS transistor in a structure measuring the temperature of heating elements of an ink-jet printhead. In the drawing, reference number 21 indicates a silicon substrate, reference number 22 indicates a field oxide, reference number 23 indicates a thermal oxide, reference number 24 indicates a polysilicon gate, reference number 27 indicates a resistive layer, and reference number 28 indicates a conductive layer. In the prior-art method, MOS driving elements are first formed on the ink-jet chip. One terminal of the ink-jet circuit connects to the drain of MOS transistor and another terminal connects to the driving voltage, so that a voltage is applied across the heating elements when a high voltage is applied to the gate of MOS transistor. Thus, an ink droplet is ejected from the channel nozzle. In order to provide a good printing quality, this kind of ink-jet printhead utilizes temperature-sensing feedback, so as to properly drive the resistors, also known as heaters, to produce uniform sized ink droplets.
It is possible to form a temperature-sensing device near each individual heating element in a prior-art ink-jet printhead. However, a large amount of terminals equal to the number of the heating elements have to be provided on the ink-jet chip to connect to the printer circuit. This is impractical for an ink-jet printhead having several ten to even several hundred of heating elements.
Accordingly, the object of this invention is to provide a method and a structure for measuring the temperature of heating elements of an ink-jet printhead, which can precisely measure the temperature of each individual heating element on the ink-jet printhead.
To achieve the above object, an extra metal layer or semiconductor layer is formed on the ink-jet chip having driving elements to precisely measure the temperature of each individual heating element. The metal layer or semiconductor layer can be wound under or near the heaters and connects to the driving elements. By controlling the line width, the method and structure can be used to measure the-temperature of any heating element while the ink droplet is ejected, wherein only one terminal need to be connected to the printer on the ink-jet chip. Thus, the temperature-sensing signal of any heating element can be transmitted to the printer. By using this technology, the size of ink droplets ejected from each individual heating element is uniform, thereby achieving high-quality printing.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.